Adaptive real-time optimization control

ABSTRACT

Systems and methods to regulate the amount of outdoor air that is introduced into a building. These systems and methods determine the mechanical load requirements based on adaptive control functionality. These systems and methods utilize extremum seeking control logic to vary the flow of outdoor air into the building in response to these load determinations.

BACKGROUND

The present application relates to handling units of a heating,ventilation and air conditioning system, and more particularly toregulating the amount of outdoor air that is introduced into the systemin order to reduce the amount of mechanical heating and coolingrequired.

FIG. 2 conceptually illustrates a single duct air-handling unit (AHU) 10of a heating, ventilation and air conditioning (HVAC) system whichcontrols the environment of a room 12 in a building. Air from room 12 isdrawn into a return duct 14 from which some of the air flows through areturn damper 16 to a supply duct 18. Some of the return air may beexhausted outside the building through an outlet damper 20 andreplenished by fresh outdoor air entering through an inlet damper 22. Aminimum amount of fresh outdoor air entering the system for properventilation within the building is typically required by building codes.The dampers 16, 20, and 22 are opened and closed by actuators which areoperated by a controller 24 to control the ratio of return air to freshoutdoor air. The mixture of return air and fresh outdoor air is forcedby a fan 25 through a cooling coil 26 and a heating coil 28 before beingfed into room 12.

Controller 24 also operates a pair of valves 27 and 29 that regulate theflow of chilled fluid through the cooling coil 26 and the flow of heatedfluid through the heating coil 28, depending upon whether thecirculating air needs to be cooled or heated. These coils 26 and 28provide “mechanical” heating and cooling of the air and are referred toherein as “mechanical temperature control elements.” The amount ofcooling or heating energy that is required to be provided by mechanicaltemperature control elements is referred to herein as a “mechanicalload” of the HVAC system.

Sensors 30 and 32, respectively, measure the temperature and humidity ofthe outdoor air and provide signals to controller 24. Another pair ofsensors 34 and 36, respectively, measure the temperature and humidity ofthe air in return duct 14. Additional temperature sensors 38 and 39 arelocated in the outlet of supply duct 18 and in room 12.

Controller 24 executes a software program that implements an air sideeconomizer function that uses outdoor air to reduce the mechanicalcooling requirements for air-handling unit 10. There are three air sideeconomizer control strategies that are in common use: temperature,enthalpy, and temperature and enthalpy. These strategies controltransitions between two air circulation modes: minimum outdoor air withmechanical cooling and maximum outdoor air with mechanical cooling.

In temperature economizer control, an outdoor air temperature iscompared to the return temperature or to a switch-over thresholdtemperature. If mechanical cooling is required and the outdoor airtemperature is greater than the return air temperature or theswitch-over threshold temperature, then a minimum amount of outdoor airrequired for ventilation (e.g. 20% of room supply air) entersair-handling unit 10. If mechanical cooling is required and the outdoorair temperature is less than the return temperature or a switch overthreshold temperature, then a maximum amount of outdoor air (e.g. 100%)enters the air-handling unit 10. In this case, the outlet damper 20 andinlet damper 22 are opened fully while return damper 16 is closed.

With enthalpy economizer control, the outdoor air enthalpy is comparedwith the return air enthalpy. If mechanical cooling is required and theoutdoor air enthalpy is greater than the return air enthalpy, then theminimum amount of outdoor air required for ventilation enters theair-handling unit. Alternatively, when mechanical cooling is requiredand the outdoor air enthalpy is less than the return air enthalpy, thenthe maximum amount of outdoor air enters air-handling unit 10.

With the combined temperature and economizer control strategy, whenmechanical cooling is required and the outdoor temperature is greaterthan the return temperature or the outdoor enthalpy is greater than thereturn enthalpy, the minimum amount of outdoor air required forventilation is used. If mechanical cooling is required and the outdoortemperature is less than the return air temperature and the outdoorenthalpy is less than the return enthalpy, then the maximum amount ofoutdoor air enters air-handling unit 10. The parameters of eitherstrategy that uses enthalpy have to be adjusted to take into accountdifferent geographic regions of the country.

There are a number of different processes that can be used to regulatedampers 16, 20, and 22 to control the fraction of outdoor air, such as,direct airflow measurement method or energy and mass balance method.

The direct airflow measurement method requires sensors that measureairflow rate, which enables the fraction of outdoor air in the supplyair to be controlled with a feedback controller. Krarti, “ExperimentalAnalysis of Measurement and Control Techniques of Outdoor Air IntakeRates in VAV Systems,” ASHRAE Transactions, Volume 106, Part 2, 2000,describes several well-known methods for directly measuring the outdoorair fraction.

Alternatively, the fraction of outdoor air in the room supply air can bedetermined by performing energy and mass balances. Drees, “VentilationAirflow Measurement for ASHRAE Standard 62-1989,” ASHRAE Journal,October, 1992; Hays et al., “Indoor Air Quality Solutions andStrategies,” Mc-Graw Hill, Inc., pages 200-201, 1995; and Krarti(supra), describe methods for determining the fraction of outdoor air inthe supply air based on a concentration balance for carbon dioxide. Thefraction of outdoor air in the supply air is determined from theexpression:

$f_{oa} = \frac{C_{ra} - C_{sa}}{C_{ra} - C_{oa}}$

where C_(ra) is the carbon dioxide concentration of the return air,C_(sa) is the carbon dioxide concentration of the supply air, and C_(oa)is the carbon dioxide concentration of the outdoor air.

Performing mass balances on the water vapor and air entering and leavingthe room gives:

$f_{oa} = \frac{\omega_{ra} - \omega_{ma}}{\omega_{ra} - \omega_{oa}}$

where ω_(ra) is the humidity ratio of the return air, ω_(ma) is thehumidity ratio of the mixed air, and ω_(oa) is the humidity ratio of theoutdoor air.

Performing an energy and mass balance on the air entering and leavingthe room gives:

$f_{oa} = \frac{h_{{ra} -}h_{ma}}{h_{ra} - h_{oa}}$

where h_(ra) is the enthalpy of the return air, h_(ma) is the enthalpyof the mixed air, and h_(oa) is the enthalpy of the outdoor air.

Assuming constant specific heats for the return air, mixed air, andoutdoor air yields:

$f_{oa} = \frac{T_{{ra} -}T_{ma}}{T_{ra} - T_{oa}}$

Alternatively, an estimate of the fraction of outdoor air in the supplyair can be determined from a model of the airflow in the air-handlingunit, as described by Seem et al., in “A Damper Control System forPreventing Reverse Airflow Through The Exhaust Air Damper ofVariable-Air-Volume Air-Handling Units,” International Journal ofHeating, Ventilating, Air-Conditioning and Refrigerating Research,Volume 6, Number 2, pp. 135-148, April 2000, which reviews equations formodeling the airflow in air-handling unit 10. See also U.S. Pat. No.5,791,408. The descriptions in both documents are incorporated herein byreference. The desired damper position can be determined based on thedesired fraction of outdoor air and the airflow model, where the desireddamper position can be determined.

One-dimensional optimization is applied to the fraction of outdoor airin the supply air to determine the optimal fraction which provides theminimal mechanical cooling load. Any of several well-known optimizationtechniques may be employed, such as the ones described by Richard P.Brent in “Algorithms for Minimization without Derivatives,”Prentice-Hall Inc., Englewood Cliffs, N.J., 1973, or Forsythe, Malcolm,and Moler in “Computer Methods for Mathematical Computations,” PrenticeHall, Englewood Cliffs, N.J., 1977. Alternatively, the “fminband”function contained in the Matlab software package available from TheMathworks, Inc., Natick MA 01760 U.S.A., may be used to find the optimalfraction of outdoor air.

These control strategies have assumed that the reference value or systemoptimal performance level was given. The reference value is typicallydetermined by a sensor. The reference value or optimal operatingconditions for a HVAC system is difficult to determine under variousdynamic parameters. One problem with economizer control is the accuracyof the sensors. Humidity sensing elements can be inaccurate andunstable, which causes the economizer cycle to operate inefficiently. Itwould be advantageous to provide an alternative control system thatminimized the need for sensors. Further, it would be advantageous toprovide an alternative control strategy were the reference value isunknown. It would also be advantageous to provide a system that uses anextremum seeking controller to enhance system performance.

SUMMARY

One embodiment relates to a system and method, which regulates an amountof outdoor air that is introduced into a building and operates amechanical temperature control device that varies temperature in thebuilding. The system and method monitors a heating control signal, adamper control signal, a cooling control signal and an outdoor airtemperature in a first state. The system and method performs a statecomparison of the heating control signal, the damper signal, the coolingsignal and the outdoor air temperature to a predetermined range ofvalues and transitions into a second state based on the secondcomparison.

Another embodiment relates to a system and method, which regulates anamount of outdoor air that is introduced into a building and operates amechanical temperature control device that varies temperature in thebuilding. The system and method monitors a heating control signal, adamper control signal, a cooling control signal, an outdoor airtemperature and an outdoor air humidity in a first state. The system andmethod computes an outdoor air enthalpy, a state a enthalpy, a state benthalpy, an outdoor air specific volume, a state b specific volume anda state a temperature. The system and method performs a state comparisonof the heating control signal, the damper signal, the cooling signal,the outdoor air temperature, the outdoor air enthalpy, the state aenthalpy, the state b enthalpy, the outdoor air specific volume, thestate b specific volume and the state a temperature to a predeterminedrange of values and transitions into a second state based on the secondcomparison.

Yet another embodiment relates to a space conditioning device includingan air-handling unit. The air-handling unit is coupled to a means forcooling an air and a means for heating an air. The space conditioningdevice includes a means for controlling the space conditioning deviceand at least one means for controlling a flow of the air coupled to themeans for controlling the space conditioning device and the air-handlingunit. The means for controlling the space conditioning device isconfigured to control the at least one means for controlling the flow ofthe air utilizing extremum logic

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an isometric view of a building according to an exemplaryembodiment with an HVAC system including an air handling unit.

FIG. 2 is a diagram of an air-handling unit in a HVAC system, accordingto an exemplary embodiment.

FIG. 3 is a state diagram of a finite state machine with four operatingstates that are implemented in the controller of the air-handling unitin FIG. 1, according to an exemplary embodiment.

FIG. 4 is an exemplary psychometric chart depicting operation of thefour states in FIG. 3 for a specific set of environmental conditions,according to an exemplary embodiment.

FIG. 5 is a state diagram of a finite state machine with five operatingstates that are implemented in the controller of the air-handling unitin FIG. 2, according to an exemplary embodiment.

FIG. 6 is an exemplary psychometric chart depicting operation of thefive states in FIG. 3 for a specific set of environmental conditions,according to an exemplary embodiment.

FIG. 7 is an exemplary psychometric chart that shows states of controlwith return conditions of 75° F. and 50% relative humidity, according toan exemplary embodiment.

FIG. 8 is an exemplary psychometric chart that shows the regions ofoptimal control and lines for transitioning between states for a coolingcoil model utilizing an ideal coil, according to an exemplaryembodiment.

FIG. 9 is an exemplary psychometric chart depicting transition lines forreturn conditions of 75° F. and 60% relative humidity and 72° F. and 40%relative humidity.

FIG. 10 is another exemplary state diagram of a finite state machinewith five operating states that is implemented in the controller of theair-handling unit in FIG. 2, according to an exemplary embodiment.

FIG. 11 is an exemplary psychometric chart depicting operation of thefive states for a specific set of environmental conditions, according toan exemplary embodiment.

FIG. 12 is a diagram of an extremum seeking control system, according toan exemplary embodiment.

FIG. 13 is another diagram of an extremum seeking control system,according to an exemplary embodiment.

FIG. 14 is a diagram of a HVAC system in which an extremum seekingcontroller has been incorporated, according to an exemplary embodiment.

FIG. 15 is a state diagram of a finite state machine with five operatingstates that are implemented in the controller of the air-handling unit,according to an exemplary embodiment.

FIG. 16 shows the extremum-controlled HVAC system tracking the optimalsolution, according to an exemplary embodiment.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Before beginning the detailed description of exemplary embodiments,several general comments are warranted about the applicability and thescope of the present invention.

Although the description below contains many specificities, thesespecificities are utilized to illustrate some of the preferredembodiments of this invention and should not be construed as limitingthe scope of the invention. The scope of this invention should bedetermined by the claims, their legal equivalents and the fact that itfully encompasses other embodiments, which may become apparent to thoseskilled in the art. A method or device does not have to address each andevery problem to be encompassed by the present invention. Allstructural, chemical, and functional equivalents to the elements of thebelow-described invention that are known to those of ordinary skill inthe art are expressly incorporated herein by reference and are intendedto be encompassed by the present claims. A reference to an element inthe singular is not intended to mean one and only one, unless explicitlyso stated, but rather it should be construed to mean at least one. Noclaim element herein is to be construed under the provisions of 35U.S.C. § 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for.” Furthermore, no element, component, ormethod step in the present disclosure is intended to be dedicated to thepublic, regardless of whether the element, component, or method step isexplicitly recited in the claims.

FIG. 1 shows a building 5 with an air-handling unit 10 according to anexemplary embodiment. Air handling unit 10 is part of a heating,ventilation and air conditioning (HVAC) system which controls theenvironment of a room 12 in a building 5.

FIG. 2 shows air-handling unit controller 24, which utilizes software tocontrol the air flow rate. In an exemplary embodiment, the softwareconfigures the controller as a finite state machine that has four statesdepicted in FIG. 3. In FIG. 3, a state diagram for controlling an airside economizer for air-handling unit 10 with only an outdoorair-temperature sensor for controlling the air side economizer is shown.

A transition occurs from one state to another state, as indicated by thearrows, when a specified condition or set of conditions occurs. In anexemplary embodiment, the operational data of the air-handling unit ischecked when the controller is in a given state to determine whether adefined transition condition exists. A number of the transitionconditions are specified in terms of the control being “saturated” inthe present state. The term saturated may be a specific time interval,temperature condition, supply air condition and/or return air condition.In an exemplary embodiment, saturation occurs when controller 24 remainsin a given operating mode for a predetermined period of time withoutbeing able to adequately control the environment of the building. Forexample, saturation occurs in a mechanical cooling mode when the systemis unable to cool the room to the desired temperature within areasonable amount of time.

In State 1, valve 29 for heating coil 28 is controlled to modulate theflow of hot water, steam, or electricity to the heating coil, therebycontrolling the amount of energy transferred to the air. In a largeair-handling system with a supply air temperature sensor, the supply airtemperature is controlled to maintain the setpoint. In a smallair-handling system without a supply air temperature sensor, the roomtemperature is controlled directly to maintain the setpoint. Dampers 16,20, and 22 are positioned for a minimum flow rate of outdoor air andthere is no mechanical cooling, (i.e. chilled water valve 27 is closed).The minimum flow rate of outdoor air is the least amount required forsatisfactory ventilation in the room, for example 20% of the airsupplied to the room is outdoor air. The condition for a transition toState 2 is defined by the heating control signal being saturated in the“No Heat Mode.” In an exemplary embodiment, the saturation may occurwhen valve 29 of heating coil 28 remains closed for a defined period oftime (i.e. heating of the supply air is not required during thatperiod). This transition condition can result from the outdoortemperature rising to a point at which the interior of the room does notneed mechanical heating.

In an exemplary embodiment for State 2, dampers 16, 20, and 22 alone areused to control the supply air temperature in supply duct 18 (i.e. nomechanical heating or cooling). In this State, the amount of outdoor airthat is mixed with the return air from the room is regulated to heat orcool the air being supplied to room 12. Because there is no heating ormechanical cooling, the inability to achieve the setpoint temperatureresults in a transition to either State 1 or 3. In an exemplaryembodiment, a transition occurs to State 1 for mechanical heating wheneither: (i) for a defined period of time the flow of outdoor air is lessthan that required for proper ventilation or (ii) the outdoor air inletdamper 22 remains in the minimum open position for a given period oftime. In an exemplary embodiment, the finite state machine makes atransition from State 2 to State 3 for mechanical cooling upon thedamper control being saturated in the maximum outdoor air position (e.g.100% of the air supplied to the room is outdoor air).

In an exemplary embodiment for State 3, chilled water valve 27 forcooling coil 26 is controlled to modulate the flow of chilled water andcontrol the amount of energy removed from the air. At this time, dampers16, 20, and 22 are positioned to introduce a maximum amount of outdoorair into AHU 10. There is no heating in this State. In an exemplaryembodiment, a transition occurs to State 2 when the mechanical coolingdoes not occur for the given period of time (i.e. the cooling control issaturated in the no cooling mode).

In the start-up or initial State, the outdoor temperature T_(out) ismeasured and compared with various thresholds to determine the initialcontrol State. In an exemplary embodiment, the system transitions fromthe initial control State to one of four States (i.e. State 1 to State4) based on T_(out). These transitions are detailed in the followingfour paragraphs.

An exemplary embodiment is shown in FIGS. 2 and 3. When T_(out) is lessthan 35° F., the system transitions into State 1. In State 1, heatingwith minimum outdoor air is implemented. State 1 controls the supply airtemperature by modulating the amount of heat supplied from heating coils28. Dampers 16, 20, and 22 are controlled for minimum ventilation. In anexemplary embodiment, a transition to State 2 occurs after the heatingcontrol signal has been at its minimum value (no heat position) for fiveminutes.

In this exemplary embodiment, when T_(out) is less than 53° F. andgreater than, or equal to, 35° F., the system transitions to State 2. InState 2, the system is utilizing outdoor air to provide free cooling tothe system. State 2 controls the supply air temperature by modulatingdampers 16, 20, and 22 to adjust the mixing of outdoor air with returnair. In an exemplary embodiment, a transition to State 1 occurs afterdampers 16, 20, and 22 have been controlled for minimum ventilation forfive minutes. In an exemplary embodiment, a transition to State 3 occursafter dampers 16, 20, and 22 have been controlled to supply 100% outdoorair for five minutes.

In this exemplary embodiment, when T_(out) is less than 63° F. andgreater than, or equal to, 53° F., the system transitions to State 3. InState 3, the system provides mechanical cooling with 100% outdoor air.State 3 controls the supply air temperature by modulating the flow rateof chilled water or refrigerant through cooling coil 26. Dampers 16, 20,and 22 are set to allow 100% outdoor air to enter air-handling unit 10.In an exemplary embodiment, a transition to State 2 occurs after thecontrol signal for mechanical cooling has been at a no-cooling value forfive minutes. In an exemplary embodiment, a transition to State 4 occursafter the outdoor temperature is greater than, or equal to, 63° F. forfive minutes.

In this exemplary embodiment, when T_(out) is greater than or equal to63° F., the system transitions to State 4. In State 4, mechanicalcooling with self-optimizing controls the outdoor air dampers. State 4controls the supply air temperature by modulating the flow rate ofchilled water or refrigerant through cooling coil 26. Self-optimizingcontrol is used to determine dampers' 16, 20, and 22 positions thatminimize the amount of mechanical cooling. Ventilation requirements areused to calculate a lower limit on the amount of outdoor air in thesupply duct 18. In an exemplary embodiment, a transition to State 3occurs after the outdoor temperature is less than 63° F. for fiveminutes.

It should be noted that these threshold temperatures can be varied forgeographic locations, client preferences, system configurations,maintenance requirements and/or system designs.

In another exemplary embodiment, the software configures controller 24as a finite state machine that has five states depicted in FIG. 5. InFIG. 5, a state diagram for controlling air side economizer forair-handling unit 10 with return air, outdoor air temperature andrelative humidity sensors is shown. In the initial state, the outdoortemperature T_(out) and relative humidity φ_(out) are measured. Standardthermodynamic procedures are used to calculate the enthalpy h_(out) andspecific volume ν_(out) of outdoor air from the temperature and relativehumidity measurements. The return air conditions T_(r) and estimates ofthe temperature T_(error) and relative humidity φ_(error) sensor errorsare used to calculate two new thermodynamic States (State a and b) asfollows:

T _(a) =T _(r) −T _(error)

φ_(a)=φ_(r)−φ_(error)

T _(b) =T _(r) +T _(error)

φ_(b)=φ_(r)+φ_(error)

The enthalpies (h_(a) and h_(b)) for States a and b are determined fromthe temperature and relative humilities and standard thermodynamiccalculations. The specific volume ν_(b) of State b is also determined.In an exemplary embodiment, the system transitions from the initialcontrol State to one of five states (i.e. State 1 to State 5) based onT_(a), T_(out), h_(b), h_(b), h_(out), ν_(out), and ν_(b). Thesetransitions are detailed in the following five paragraphs.

An exemplary embodiment is shown in FIGS. 5 and 6. When T_(out) is lessthan 35° F., the system transitions into State 1. In State 1, heatingwith minimum outdoor air is implemented. State 1 controls the supply airtemperature by modulating the amount of heat supplied from heating coils28. Dampers 16, 20, and 22 are controlled for minimum ventilation. In anexemplary embodiment, a transition to State 2 occurs after the heatingcontrol signal has been at its minimum value (no heat position) for fiveminutes.

In this exemplary embodiment, when T_(out) is less than 53° F. andgreater than, or equal to, 35° F., the system transitions to State 2. InState 2, the system is utilizing outdoor air to provide free cooling tothe system. State 2 controls the supply air temperature by modulatingdampers 16, 20, and 22 to adjust the mixing of outdoor air with returnair. In an exemplary embodiment, a transition to State 1 occurs afterdampers 16, 20, and 22 have been at minimum ventilation for fiveminutes. In an exemplary embodiment, a transition to State 3 occursafter dampers 16, 20, and 22 have been controlled to supply 100% outdoorair for five minutes.

In this exemplary embodiment, when T_(out) is greater than, or equal to,53° F. and less than, or equal to, T_(a) while h_(out) is less thanh_(a), the system transitions to State 3. In State 3, the systemutilizes mechanical cooling with 100% outdoor air. State 3 controls thesupply air temperature by modulating the flow rate of chilled water orrefrigerant through cooling coil 26. Dampers 16, 20, and 22 are set toallow 100% outdoor air to enter air-handling unit 10. In an exemplaryembodiment, a transition to State 2 occurs after the control signal formechanical cooling has been at a no-cooling value for five minutes. Inan exemplary embodiment, a transition to State 4 occurs if either of thefollowing conditions is true for five minutes: (i) the outdoor enthalpyh_(out) is greater than the enthalpy h_(a) of State h_(a), or (ii) theoutdoor temperature T_(out) is greater than temperature T_(a) of Statea.

In this exemplary embodiment, when h_(out) is less than, or equal to,h_(b) and T_(out) is greater than, or equal to, T_(a) or h_(out) isgreater than, or equal to, h_(a) and ν_(out) is less than, or equal to,ν_(b), the system transitions to State 4. In State 4, the systemutilizes mechanical cooling with self-optimizing control to controldampers 16, 20, and 22. State 4 controls the supply air temperature bymodulating the flow rate of chilled water or refrigerant through coolingcoil 26. Self-optimizing control is used to determine dampers' 16, 20,and 22 positions that minimizes the amount of mechanical cooling.Outdoor air flow rate lower limit is the minimal ventilationrequirement. In an exemplary embodiment, a transition to State 3 occursafter the following two conditions are true for five minutes: (i)outdoor enthalpy h_(out) is less than, or equal to, the enthalpy h_(a)of State a, and (ii) the outdoor temperature T_(out) is less than, orequal to, the temperature T_(a) of State a. In an exemplary embodiment,a transition to State 5 occurs after the following two conditions aretrue for five minutes: (i) outdoor enthalpy h_(out) is greater than theenthalpy h_(b) of thermodynamic State b, and (ii) the specific volumeν_(out) of outdoor air is greater than the specific volume ν_(b) ofthermodynamic State b.

In this exemplary embodiment, when h_(b) is less than h_(out) and ν_(b)is less than ν_(out), the system transitions to State 5. In State 5, thesystem utilizes mechanical cooling with minimum outdoor air required forventilation. State 5 controls the supply air temperature by modulatingthe flow rate of chilled water or refrigerant through cooling coil 26.Dampers 16, 20, and 22 are controlled to provide the minimum outdoor airrequired for ventilation. In an exemplary embodiment, a transition toState 4 occurs if either of the following conditions is true for fiveminutes: (i) the enthalpy of the outdoor enthalpy h_(out) is less than,or equal to, the enthalpy h_(b) of thermodynamic State b, or (ii) thespecific volume ν_(out) of outdoor air is less than, or equal to, thespecific volume ν_(b) of thermodynamic State b.

It should be noted that these threshold temperatures, enthalpies andspecific volumes can be varied for geographic locations, clientpreferences, system configurations, maintenance requirements and/orsystem designs.

Simulations were performed for an ideal coil and air-handling unit 10that had a minimum fraction of outdoor air to supply air of 20%, and areturn temperature of 75° F. and relative humidity of 50%. FIGS. 6 and 7shows outdoor air conditions where the self-optimizing control willtransition between the following three fractions of outdoor air: (i) 20%outdoor air, (ii) between 20 and 100% outdoor air, and (iii) 100%outdoor air. FIG. 8 shows results for an ideal coil. A person skilled inthe art will notice that the optimal regions of control are dependent onthe type of coil. In an exemplary embodiment, the regions may bedependent on the return air conditions, setpoint for the supply airtemperature, and minimum fraction of outdoor air to supply air requiredfor ventilation.

FIG. 9 shows the outdoor air conditions and corresponding control stateregions on a psychometric chart for the following two different returnconditions: (i) 75° F. and 60% relative humidity, and (ii) 72° F. and40% relative humidity. The point r₁ is for return conditions of 75° F.and 60% relative humidity, and the corresponding thermodynamic statesfor determining transitions are a₁ and b₁. The point r₂ is for returnconditions of 72° F. and 40% relative humidity, and the correspondingthermodynamic states for determining transitions are a₂ and b₂. FIG. 9shows how the transition lines between control states moves as thereturn conditions change from r₁ to r₂.

Referring to FIG. 10, an alternative embodiment five state system isshown. In State 1, valve 29 for heating coil 28 is controlled tomodulate the flow of hot water, steam, or electricity to the heatingcoil, thereby controlling the amount of energy transferred to the air.This maintains the room temperature at the setpoint. Dampers 16, 20, and22 are positioned for a minimum flow rate of outdoor air and there is nomechanical cooling (i.e. chilled water valve 27 is closed). The minimumflow rate of outdoor air is the least amount required for satisfactoryventilation in the room. For example, 20% of the air supplied to theroom is outdoor air. The condition for a transition to State 2 isdefined by the heating control signal being saturated in the no heatposition. Such saturation occurs when valve 29 of heating coil 28remains closed for a defined period of time (i.e. heating of the supplyair is not required during that period). This transition condition canresult from the outdoor temperature rising to a point at which theinterior of the room does not need mechanical heating.

In State 2, dampers 16, 20, and 22 alone are used to control the supplyair temperature in supply duct 18 (i.e. no mechanical heating orcooling). In State 2 the amount of outdoor air that is mixed with thereturn air from the room is regulated to heat or cool the air beingsupplied to room 12. Because there is no heating or mechanical cooling,the inability to achieve the setpoint temperature results in atransition to either State 1 or 3. A transition occurs to State 1 formechanical heating when either for a defined period of time the flow ofoutdoor air is less than that required for proper ventilation or outdoorair inlet damper 22 remains in the minimum open position for a givenperiod of time. The finite state machine makes a transition from State 2to State 3 for mechanical cooling upon the damper control beingsaturated in the maximum outdoor air position (e.g. 100% of the airsupplied to the room is outdoor air).

In State 3, chilled water valve 27 for cooling coil 26 is controlled tomodulate the flow of chilled water and control the amount of energyremoved from the air. At this time, dampers 16, 20, and 22 arepositioned to introduce a maximum amount of outdoor air into AHU 10.There is no heating in this State. A transition occurs to State 2 whenthe mechanical cooling does not occur for the given period of time (i.e.the cooling control is saturated in the no cooling mode).

Transitions between States 3 and 4 are based on estimates of the loadthat is exerted on cooling coil 26 when outdoor air flows into AHU 10 atminimum and with maximum flow rates. Thus, in both of those State 3 andState 4 the air-handling controller performs those estimations. Thethree principal steps involved in the estimation process are: (1)determine the mixed air conditions from the fraction of outdoor air inthe room supply air and from the outdoor and return air conditions, (2)determine the desired air temperature after cooling coil 26 from thesetpoint temperature and an estimate of the heat gain from fan 25, and(3) estimate the load exerted on the mechanical cooling coil 26. SinceStates 3 and 4 control cooling of the room air, the particularmechanical temperature control element for which the load is beingestimated in cooling coil 26. However, one skilled in the art willappreciate that the present inventive concept may also be employed inheating states where mechanical temperature control element is heatingcoil 28.

The mixed air humidity ratio ω_(m′) and enthalpy h_(m′) are determinedfrom the expressions:

$\omega_{m} = {{\frac{{\overset{.}{m}}_{o}}{{\overset{.}{m}}_{s}}w_{o}} + {( {1 - \frac{{\overset{.}{m}}_{o}}{{\overset{.}{m}}_{s}}} )\omega_{r}}}$$h_{m} = {{\frac{{\overset{.}{m}}_{o}}{{\overset{.}{m}}_{s}}h_{o}} + {( {1 - \frac{{\overset{.}{m}}_{o}}{{\overset{.}{m}}_{s}}} )h_{r}}}$

where ω_(ó) and ω_(r) are the outdoor air and return air humidityratios, respectively; m_(ó) and m_(s) are the mass flow rate of theoutdoor air and supply air, respectively; and h_(ó′) and h_(r′) are theenthalpy of the outdoor air and return air, respectively. Therefore, theterm m_(o)/m_(s) represents the fraction of outdoor air in the air beingsupplied to room 12, (i.e. 0.20 or 1.00 for the State machine). Thehumidity ratios and enthalpy for the outdoor air and return air aredetermined from temperature and relative humidity measurements providedfrom sensors 30, 32, 34, and 36 and by psychometric equations providedby the 1997 ASHRAE Handbook—Fundamentals, Chapter 6, American Society ofHeating, Refrigerating and Air-Conditioning Engineers, 1997; and ASHRAE,Psychometrics—Theory and Practice, American Society of Heating,Refrigerating, and Air-Conditioning Engineers, ISBN 1-883413-39-7,Atlanta, Ga., 1996.

The air temperature after cooling coil 26 is determined from thesetpoint temperature for the supply air and an estimate of thetemperature rise across fan 25 as determined from the equation:

${T_{s} - T_{c}} = \frac{P_{s} - P_{c}}{\rho \; C_{p}\eta_{o}}$

where ρ is the air density, c_(p) is the constant pressure specificheat, η_(o) is the overall efficiency of the components in the duct.P_(S)−P_(C) equals the pressure rise across the fan, and T_(s) and T_(c)are the supply air and chilled air temperature, respectively. Thechilled air temperature is the bulk air temperature after cooling coil26. The overall efficiency can be determined by multiplying theefficiencies of the components in the duct. If the fan, drive, and motorare all in the duct, then the overall efficiency η_(o) is determinedfrom:

η_(o)=η_(fan)η_(drive)η_(motor)

where η_(fan) is the fan efficiency, η_(drive) is the efficiency of thedrive, η_(motor), is the motor efficiency. The fan efficiency is theratio of work output to mechanical input, the drive efficiency is theratio of electrical output to input, and the motor efficiency is theratio of mechanical output to electrical input.

A number of different models can used to estimate the load exerted oncooling coil 26. However, a preferred technique determines the coolingload from a bypass factor approach as described by Kuehn et al., ThermalEnvironmental Engineering, Prentice-Hall Inc., Upper Saddle River, N.J.,1998.

In that technique, a determination first is made whether cooling coil 26is dry. The following equation is employed to determine the temperatureat which the coil transitions between a dry condition and a partiallywet condition:

T*=βT _(m)+(1−β)T _(dew,m)

where T* is the transition temperature, β is the coil bypass factor,T_(m) is the mixed air temperature, and T_(dew,m) is the dew pointtemperature of the mixed air. The mixed air temperature and dew pointtemperature can be determined from previous equations, and thepsychometric equations presented in ASHRAE Handbook— Fundamentals,supra. If the cool air temperature is greater than the transitiontemperature, cooling coil 26 is dry, otherwise cooling coil 26 ispartially wet or wet.

If cooling coil 26 is dry, then the cooling load is derived from theexpression:

$\frac{{\overset{.}{Q}}_{c}}{\overset{.}{m_{a}}} = {h_{m} - h_{c}}$

where {dot over (Q)}′_(C) is the cooling load, m_(a) is the mass flowrate of dry air, and h_(m) and h_(C) are the enthalpy of the mixed airand cooled air, respectively. The enthalpies are determined from themixed air temperature and relative humidity, the cooled air temperature,the psychometric equations presented in 1997 ASHRAEHandbook—Fundamentals, supra and the following equation:

ω_(c)=ω_(m)

If cooling coil 26 is not dry, then the cooling load is derived from theexpression:

$\frac{{\overset{.}{Q}}_{c}}{\overset{.}{m_{a}}} = {{( {1 - \beta} )( {h_{m} - h_{d}} )} - {{h_{w}( {1 - \beta} )}( {\omega_{m} - \omega_{d}} )}}$

where β is the coil bypass factor, h_(d) and w_(c) are the enthalpy andhumidity ratio of the saturated air, and h_(w) is the enthalpy ofcondensate. The dew point temperature T_(dew) for the saturated air isdetermined from:

$T_{dew} = \frac{T_{c} - {\beta \; T_{m}}}{1 - \beta}$

The enthalpy and the humidity ratio for the saturated air is determinedfrom the dew point temperature and the ASHRAE psychometric equations.When controller 24 is operating in State 3, an estimate of the coolingload with the minimum and maximum flow rates of outdoor air are derived.

In an exemplary embodiment, a transition can occur from State 3 toeither State 4 or 5 depending upon the values of these cooling loadestimates. The transition occurs to State 4 when the estimated coolingload with maximum outdoor air {dot over (Q)}_(MAX) is less than apredetermined percentage (i.e. 1-10 percent) of the estimated coolingload with minimum outdoor air {dot over (Q)}_(MIN) for a period ofthirty seconds. This predetermined percentage allows the control systemto determine the position of the system on the psychometric chart. Thetransition occurs to State 5 when the estimated cooling load withmaximum outdoor air {dot over (Q)}_(MAX) is greater than a predeterminedpercentage of the estimated cooling load with minimum outdoor air {dotover (Q)}_(MIN) for a period of thirty seconds. In State 5, the optimalfraction of outdoor air {dot over (Q)}_(OPT) is determined by theextremum seeking control system.

In State 4, cooling coil 26 is active to apply mechanical cooling to theair while dampers 16, 20, and 22 are set in the minimum outdoor airpositions. A transition occurs to State 3 when the estimated coolingload with minimum outdoor air {dot over (Q)}_(MIN) is less than apredetermined percentage (i.e. 1-10 percent) greater than the estimatedcooling load of the maximum outdoor air {dot over (Q)}_(MAX) for aperiod of thirty seconds. A transition occurs from State 4 to State 5when the estimated cooling load with minimum outdoor air {dot over(Q)}_(MIN) is greater than a predetermined percentage of the estimatedcooling load of the maximum outdoor air {dot over (Q)}_(MAX) for aperiod of thirty seconds. In State 5, the optimal fraction of outdoorair {dot over (Q)}_(OPT) is determined by the extremum seeking controlsystem.

In State 5, dampers 16, 20, and 22 are extremum controlled to modulatethe flow of chilled water to remove energy from the circulating air. Atthis time, the positions of dampers 16, 20, and 22 are dynamicallyvaried to introduce outdoor air into the system. This dynamic systemintroduces an amount of outdoor air that approaches the optimal fractionof outdoor air for the system. A transition occurs to State 3 when thecooling load derived by extremum control with optimal fraction ofoutdoor air {dot over (Q)}_(OPT) is greater than, or equal to, theestimated cooling load with the maximum outdoor air {dot over (Q)}_(MAX)for a period of thirty seconds. A transition occurs from State 5 toState 4 when the cooling load derived by extremum control with optimalfraction of outdoor air {dot over (Q)}_(OPT) is greater than, or equalto, the estimated cooling load with minimum outdoor air {dot over(Q)}_(MIN) for a period of thirty seconds.

FIG. 11 shows that a transition from State 3 to State 4 occurs after theHVAC system reaches a transition line₃₄ 42 on the psychometric chart.The transition from State 3 to State 5 occurs after the HVAC systemreaches a transition line₃₅ 40 on the psychometric chart. The transitionfrom State 4 to State 5 occurs after the HVAC system reaches atransition line₄₅ 48 on the psychometric chart.

FIG. 11 also shows that a transition from State 5 to State 4 occursafter the HVAC system reaches a transition line₅₄ 50 on the psychometricchart. The transition from State 5 to State 3 occurs after the HVACsystem reaches a transition line₅₃ 46 on the psychometric chart. Thetransition from State 4 to State 3 occurs after the HVAC system reachesa transition line₄₃ 44 on the psychometric chart.

In FIGS. 12-14, an extremum control system 62 is shown. Extremum controlis the tracking of a varying maximum and minimum parameter. Therelationship between inputs and outputs in a static response curve isnonlinear in extremum control system 62. The extremum controller findsthe optimum operating point and tracks it under varying conditions(e.g., changes in temperature, humidity ratio, etc.).

In FIG. 12, a basic extremum control system 52 is shown. The process canwork in either an open loop or closed loop control system. A searchalgorithm 54 continually modifies the output of the process to approachthe extremum despite a change in the process 56 or an influence ofdisturbances 58. The process communicates search algorithm 54 to plant74. Search algorithm 54 determines a setpoint for the system. Inaddition to search algorithm 54 communicating the setpoint to a plant74, plant 74 also receives change in the process 56 signal from theprocess. Plant 74 is configured to use either signal to modify theprocess. In an exemplary embodiment, plant 74 may use change in theprocess 56 signal from the process to provide a command 60 to theprocess to move the system towards the extremum. In another exemplaryembodiment, plant 74 may use search algorithm 54 signal from to providecommand 60 to the process to move the system towards the extremum.

In FIG. 13, a basic extremum seeking static map 62 is shown. Where y isthe output to be minimized; f* is the minimum of the map; f″ is thesecond derivative (positive—f(θ) has a min.); θ* is the unknownparameter; θ′ is the estimate of θ*; k is the adaptation gain (positive)of the integrator 1/s (where s is the variable that results from Laplacetransform); a is the amplitude of the probing signal; ω is the frequencyof the probing signal; h is the cut-off frequency of the washoutfilter; + is “modulation” by summation and X is “demodulation” bymultiplication. Where a washout filter is given by:

$h = \frac{s}{s + h}$

Extremum control system 62 starts with an estimate of the unknownparameter θ*. The control system uses this parameter to determine theoptimum operating point and to track the optimum operating point as itvaries. The output to be minimized y is transmitted to a washout filter64. Washout filter 64 screens y and transmits the screened y to amultiplier 66. Multiplier 66 transmits ξ to an adaptation gain filter68. Adaptation gain filter 68 transmits an estimate of θ* to ansummation 70, which transmits a control signal to plant 74. In anexemplary embodiment, the algorithm for the extremum seeking system is asingle parameter system. It is noted that the algorithm may have severalor a plurality of parameters.

In FIG. 14, an extremum seeking control system 76 for a HVAC system isshown. Extremum seeking control system 76 includes a heat exchanger 80,a feedback controller 90, a high pass filter 86, a mixer 96, anintegrator 98, an amplifier 100, a damper command control 102 and amixing damper 88. Heat exchanger 80 lowers the temperature of the air.Feedback controller 90 maintains a supply air temperature 78 at asetpoint 92 by adjusting the position for chilled water valve of coolingcoil 26 (FIG. 2). Damper command control 102 maintains dampers 16, 20,and 22 between 0% and 100% outside air.

An exemplary embodiment of a finite state machine after the start upstage has been implemented, is shown in FIG. 15. In FIG. 15, a finitestate machine for controlling an air-side economizer that usesself-optimizing control (also called extremum seeking control) tocontrol dampers 16, 20, and 22 for one of the states. The numbersindicate the state number for the finite-state machine. The transitionsbetween States 3 and 4 are based on temperature T_(a) and enthalpyh_(a). The temperature and relative humidity for thermodynamic State aare determined from:

T _(a) =T _(r) −T _(error)

Ø_(a)=Ø_(r)−Ø_(error)

Where T_(r) and Ø_(r) are the temperature and relative humidity of thereturn air. T_(error) and Ø_(error) are estimates of the errors fortemperature and relative humidity sensors, respectively. Sensor errorscan be caused by being out of calibration, being in a poor location, andthe inherent accuracy of the sensor. Standard psychometric propertycalculation methods can be used to determine the enthalpy of the outdoorand State a from the temperatures and relative humidities. Thesestandard psychometric property calculation methods can be found inPsychometrics Theory and Practice by the American Society of Heating,Refrigeration and Air-Conditioning Engineers, Inc. 1996, ISBN1-883413-39-7.

Transitions between States 4 and 5 are based on enthalpies and specificvolumes of the outdoor air conditions and thermodynamic State b. Thetemperature and relative humidity for State b are determined from:

T _(b) =T _(r) +T _(error)

Ø_(b)=Ø_(r)+Ø_(terror)

The program Engineering Equation Solver (available athttp://www.mhhe.com/engcs/mech/ees/download.html) was used to determinethe regions of outdoor air conditions for the control States on apsychometric chart when the return conditions were 75° F. and 50%relative humidity (See FIG. 7). The air temperature after the coil was53° F. The temperature error (T_(error)) was 1° F. and the relativehumidity error (Ø_(error)) was 3%. FIG. 7 shows the outdoor airconditions on a psychometric chart for States 2 through 5. Thethermodynamic location of States a 106 and State b 104 will varydepending on return air conditions, which are a function of supply airconditions, thermal gains and moisture gains in the space. In FIG. 7there is no transition directly from State 3 to State 5. Therefore, thetransition line 108 is where the control system will transition fromState 3 to State 4 or vice versa. Whereas, the transition line 110 iswhere the control system will transition from State 4 to State 5 or viceversa.

Simulations for an ideal coil model were used to determine three regionsof optimal fraction of outdoor air to supply air: minimum (20%) outdoorair, between 20% and 100% outdoor air, and 100% outdoor air. (A1-dimensional search was used to find the thermodynamic conditions forthe transitions.) The simulations assumed ideal sensors. FIG. 8 showsthe three areas of outdoor air and lines for the transitions betweenstates.

FIG. 16 shows the extremum controlled system varying the percentage ofoutside from twenty percent 116 to one hundred percent 118. The optimalpercentage of outside air curve 114 is tracked by the extremumcontrolled outside air curve 112. The extremum controlled system tracksthe optimal solution, which causes some inefficiency 120 in the system.

In an exemplary embodiment, the cooling coil control signal is afunction of the load on cooling coil 26. The signal from cooling coil 26is communicated to the control circuit. The control circuit utilizingextremum seeking logic modulates dampers 16, 20, and 22.

While the exemplary embodiments illustrated in the figures and describedabove are presently preferred, it should be understood that theseembodiments are offered by way of example only. Accordingly, the presentinvention is not limited to a particular embodiment, but extends tovarious modifications that nevertheless fall within the scope of theappended claims. The order or sequence of any processes or method stepsmay be varied or re-sequenced according to alternative embodiments.

Describing the invention with figures should not be construed asimposing on the invention any limitations that may be present in thefigures. The present invention contemplates methods, systems and programproducts on any machine-readable media for accomplishing its operations.The embodiments of the present invention may be implemented using anexisting computer processors, by a special purpose computer processorfor an appropriate HVAC system, or by an analog electrical circuitincorporated for this or another purpose or by a hardwired system.

It is important to note that the construction and arrangement of theadaptive real-time optimization control system as shown in the variousexemplary embodiments is illustrative only. Although only a fewembodiments of the present inventions have been described in detail inthis disclosure, those skilled in the art who review this disclosurewill readily appreciate that many modifications are possible (e.g.,variations in sizes, dimensions, structures, shapes and proportions ofthe various elements, values of parameters, mounting arrangements, useof materials, colors, orientations, etc.) without materially departingfrom the novel teachings and advantages of the subject matter recited inthe claims. For example, elements shown as integrally formed may beconstructed of multiple parts or elements (e.g., air-handling unit), theposition of elements may be reversed or otherwise varied (e.g.,sensors), and the nature or number of discrete elements or positions maybe altered or varied (e.g., sensors). Accordingly, all suchmodifications are intended to be included within the scope of thepresent invention as defined in the appended claims. The order orsequence of any process or method steps may be varied or re-sequencedaccording to alternative embodiments. In the claims, anymeans-plus-function clause is intended to cover the structures describedherein as performing the recited function and not only structuralequivalents but also equivalent structures. Other substitutions,modifications, changes and omissions may be made in the design,operating conditions and arrangement of the exemplary embodimentswithout departing from the scope of the present inventions as expressedin the appended claims.

As noted above, embodiments include program products comprisingmachine-readable media for carrying or having machine-executableinstructions or data structures stored thereon. Such machine-readablemedia can be any available media which can be accessed by a generalpurpose or special purpose computer or other machine with a processor.By way of example, such machine-readable media can comprise RAM, ROM,EPROM, EEPROM, CD-ROM or other optical disk storage, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to carry or store desired program code in the form ofmachine-executable instructions or data structures and which can beaccessed by a general purpose or special purpose computer or othermachine with a processor. When information is transferred or providedover a network or another communications connection (either hardwired,wireless, or a combination of hardwired or wireless) to a machine, themachine properly views the connection as a machine-readable medium.Thus, any such connection is properly termed a machine-readable medium.Combinations of the above are also included within the scope ofmachine-readable media. Machine-executable instructions comprise, forexample, instructions and data which cause a general purpose computer,special purpose computer, or special purpose processing machines toperform a certain function or group of functions.

It should be noted that although the figures herein may show a specificorder of method steps, it is understood that the order of these stepsmay differ from what is depicted. Also two or more steps may beperformed concurrently or with partial concurrence. Such variation willdepend on the software and hardware systems chosen and on designerchoice. It is understood that all such variations are within the scopeof the invention. Likewise, software implementations of the presentinvention could be accomplished with standard programming techniqueswith rule based logic and other logic to accomplish the variousconnection steps, processing steps, comparison steps and decision steps.

1. A method for operating a system which regulates an amount of outdoorair that is introduced into a building and operates a mechanicaltemperature control device that varies temperature in the building, themethod comprising: monitoring a heating control signal, a damper controlsignal, a cooling control signal and an outdoor air temperature in afirst state; and performing a state comparison of the heating controlsignal, the damper signal, the cooling signal and the outdoor airtemperature to a predetermined range of values; and transitioning into asecond state based on the state comparison.
 2. The method as recited inclaim 1, further comprising: measuring the outdoor air temperature;performing a temperature comparison of the outdoor temperature to apredetermined range of temperatures; and transitioning into the firststate based on the temperature comparison.
 3. The method as recited inclaim 1, wherein an extremum seeking control circuit is used for varyingthe flow of outdoor air into the building.
 4. The method as recited inclaim 1, wherein the predetermined range of values is based on ageographic region.
 5. The method as recited in claim 1, wherein themonitoring the heating control signal, the damper control signal, thecooling control signal and the outdoor air temperature is initiated on apredetermined interval for at least the first state.
 6. The method asrecited in claim 2, wherein the predetermined range of temperatures are:less than 35° F. for a heating state; less than 53° F. and greater thanor equal to 35° F. for a free cooling state; less than 63° F. andgreater than or equal to 53° F. for a mechanical cooling state; andgreater than or equal to 63° F. for a self-optimizing state.
 7. Themethod as recited in claim 2, wherein the predetermined range oftemperatures is based on a geographic region.
 8. The method as recitedin claim 2, wherein the predetermined range of temperatures is based ona system design.
 9. The method as recited in claim 5, wherein the atleast first state comprises the first state, the second state, a thirdstate and a fourth state.
 10. The method as recited in claim 5, whereinthe at least first state comprises the first state, the second state, athird state, a fourth state and a fifth state.
 11. A method foroperating a system which regulates an amount of outdoor air that isintroduced into a building and operates a mechanical temperature controldevice that varies temperature in the building, the method comprising:monitoring a heating control signal, a damper control signal, a coolingcontrol signal, an outdoor air temperature and an outdoor air humidityin a first state; computing an outdoor air enthalpy, a state a enthalpy,a state b enthalpy, an outdoor air specific volume, a state b specificvolume and a state a temperature; performing a state comparison of theheating control signal, the damper signal, the cooling signal, theoutdoor air temperature, the outdoor air enthalpy, the state a enthalpy,the state b enthalpy, the outdoor air specific volume, the state bspecific volume and the state a temperature to a predetermined range ofvalues; and transitioning into a second state based on the statecomparison.
 12. The method as recited in claim 11, further comprising:comparing the outdoor air temperature, the state a temperature, theoutdoor air enthalpy, the state a enthalpy, the state b enthalpy, theoutdoor air specific volume and the state b specific volume to apredetermined range of values; and transitioning into the first statebased on the comparison.
 13. The method as recited in claim 11, whereinthe predetermined range of values is based on a geographic region. 14.The method as recited in claim 11, wherein an extremum seeking controlcircuit is used for varying the flow of outdoor air into the building.15. The method as recited in claim 11, wherein performing the statecomparison of the heating control signal, the damper signal, the coolingsignal, the outdoor air temperature, the outdoor air enthalpy, the statea enthalpy, the state b enthalpy, the outdoor air specific volume, thestate b specific volume and the state a temperature to a predeterminedrange of values is initiated on a predetermined interval for at leastthe first state.
 16. The method as recited in claim 12, wherein thepredetermined range of values are: the outdoor air temperature is lessthan 35° F. for a heating state; the outdoor air temperature is lessthan 53° F. and greater than or equal to 35° F. for a free coolingstate; the outdoor air temperature is greater than or equal to 53° F.and the outdoor air enthalpy is less than the state a enthalpy for amechanical cooling state; the outdoor air temperature is greater than orequal to the state a temperature and the outdoor air enthalpy is lessthan or equal to the state b enthalpy or the outdoor air enthalpy isgreater than or equal to the state a enthalpy and the outdoor airspecific volume is less than or equal to the state b specific volume fora self-optimizing state; and the outdoor air enthalpy is greater thanthe state b enthalpy and the outdoor air specific volume is greater thanthe state b specific volume for a mechanical cooling with minimumoutdoor air state.
 17. The method as recited in claim 12, wherein thepredetermined range of values is based on a system maintenance.
 18. Themethod as recited in claim 12, wherein the predetermined range of valuesis based on a system design.
 19. The method as recited in claim 12,wherein the predetermined range of values is based on a systemconfiguration.
 20. The method as recited in claim 15, wherein the atleast first state comprises the first state, the second state, a thirdstate, a fourth state and a fifth state.
 21. A space conditioning devicecomprising: an air-handling unit; a means for cooling an air coupled tothe air-handling unit; a means for heating the air coupled to theair-handling unit; a means for controlling the space conditioningdevice; at least one means for controlling a flow of the air coupled tothe means for controlling the space conditioning device and theair-handling unit; and wherein the means for controlling the spaceconditioning device is configured to control the at least one means forcontrolling the flow of the air utilizing extremum logic.
 22. The spaceconditioning device of claim 21, further comprising at least onetemperature sensor and at least one humidity sensor.
 23. The spaceconditioning device of claim 21, wherein the means for controlling thespace conditioning device monitors a heating control signal, a dampercontrol signal, a cooling control signal and an outdoor air temperaturein a first state; wherein the means for controlling the spaceconditioning device performs a state comparison of the heating controlsignal, the damper signal, the cooling signal and the outdoor airtemperature to a predetermined range of values; and transitions into asecond state based on the state comparison.
 24. The space conditioningdevice of claim 23, wherein the predetermined range of values are: lessthan 35° F. for a heating state; less than 53° F. and greater than orequal to 35° F. for a free cooling state; less than 63° F. and greaterthan or equal to 53° F. for a mechanical cooling state; and greater thanor equal to 63° F. for a self-optimizing state.